Thin laminate flooring and wood veneer flooring are usually composed of a body consisting of a 6-9 mm fibreboard, a 0.20-0.8 mm thick upper surface layer and a 0.1-0.6 mm thick lower balancing layer. The surface layer provides appearance and durability to the floorboards. The body provides stability and the balancing layer keeps the board level when the relative humidity (RH) varies during the year. The RH can vary between 15% and 90%. Conventional floorboards of the type are usually joined by means of glued tongue-and-groove joints (i.e. joints involving a tongue on a floorboard and a tongue groove on an adjoining floorboard) at the long and short sides. When laying the floor, the boards are brought together horizontally, whereby a projecting tongue along the joint edge of a first board is introduced into a tongue groove along the joint edge of the second adjoining board. The same method is used at the long side as well as the short side. The tongue and the tongue groove are designed for such horizontal joining only and with special regard to how glue pockets and gluing surfaces should be designed to enable the tongue to be efficiently glued within the tongue groove. The tongue-and-groove joint presents coacting upper and lower contact surfaces that position the boards vertically in order to ensure a level surface of the finished floor.
In addition to such conventional floors, which are connected by means of glued tongue-and-groove joints, floorboards have recently been developed which are instead mechanically joined and which do not require the use of glue. This type of mechanical joint system is hereinafter referred to as a “strip-lock system”, since the most characteristic component of this system is a projecting strip which supports a locking element.
WO 94/26999 and WO88/66151 (owner Välinge Aluminium AB) disclose a strip-lock system for joining building panels, particularly floorboards. This locking system allows the boards to be locked mechanically at right angles to as well as parallel with the principal plane of the boards at the long side as well as at the short side. Methods for making such floorboards are disclosed in EP 0958441 and EP 0958442 (owner Välinge Aluminium AB). The basic principles of the design and the installation of the floorboards, as well as the methods for making the same, as described in the four above-mentioned documents are usable for the present invention as well, and therefore these documents are hereby incorporated by reference.
In order to facilitate the understanding and description of the present invention, as well as the comprehension of the problems underlying the invention, a brief description of the basic design and function of the known floorboards according to the above-mentioned WO 94/26999 and WO 99/66151 will be given below with reference to FIGS. 1-3 in the accompanying drawings. Where applicable, the following description of the prior art also applies to the embodiments of the present invention described below.
FIGS. 3a and 3b are thus a top view and a bottom view respectively of a known floorboard 1. The board 1 is rectangular with a top side 2, an underside 3, two opposite long sides 4a, 4b forming joint edge portions and two opposite short sides 5a, 5b forming joint edge portions.
Without the use of the glue, both the long sides 4a, 4b and the short sides 5a, 5b can be joined mechanically in a direction D2 in FIG. 1c, so that they join in a joint plane F (marked in FIG. 2c). For this purpose, the board 1 has a flat strip 6, mounted at the factory, projecting horizontally from its one long side 4a, which strip extends throughout the length of the long side 4a and which is made of flexible, resilient sheet aluminium. The strip 6 can be fixed mechanically according to the embodiment shown, or by means of glue, or in some other way. Other strip materials can be used, such as sheets of other metals, as well as aluminium or plastic sections. Alternatively, the strip 6 may be made in one piece with the board 1, for example by suitable working of the body of the board 1. The present invention is usable for floorboards in which the strip is integrally formed with the body and solves special problems appearing in such floorboards and the making thereof. The body of the floorboard need not be, but is preferably, made of a uniform material. However, the strip 6 is always integrated with the board 1, i.e. it is never mounted on the board 1 in connection with the laying of the floor but it is mounted or formed at the factory. The width of the strip 6 can be about 30 mm and its thickness about 0.5 mm. A similar, but shorter strip 6′ is provided along one short side 5a of the board 1. The part of the strip 6 projecting from the joint plane F is formed with a locking element 8 extended throughout the length of the strip 6. The locking element 8 has an operative locking surface 10 facing the joint plane F and having a height of e.g. 0.5 mm. When the floor is being laid, this locking surface 10 coacts with a locking groove 14 formed in the underside 3 of the joint edge portion 4b of the opposite long side of an adjoining board 1′. The short side strip 6′ is provided with a corresponding locking element 8′, and the joint edge portion 5b of the opposite short side has a corresponding locking groove 14′. The edge of the locking grooves 14, 14′ facing away from the joint plane F forms an operative locking surface 10′ for coaction with the operative locking surface 10 of the locking element.
Moreover, for mechanical joining of both long sides and short sides also in the vertical direction (direction D1 in FIG. 1c) the board is formed with a laterally open recess 16 along one long side (joint edge portion 4a) and one short side (joint edge portion 5a). At the bottom, the recess 16 is defined by the respective strips 6, 6′. At the opposite edge portions 4b and 5b there is an upper recess 18 defining a locking tongue 20 coacting with the recess 16 (see FIG. 2a).
FIGS. 1a-1c show how two long sides 4a, 4b of two such boards 1, 1′ on an underlay 12 can be joined together by means of downward angling. FIGS. 2a-2c show how the short sides 5a, 5b of the boards 1, 1′ can be joined together by snap action. The long sides 4a, 4b can be joined together by means of both methods, while the short sides 5a, 5b—when the first row has been laid—are normally joined together subsequent to joining together the long sides 4a, 4b and by means of snap action only.
When a new board 1′ and a previously installed board 1 are to be joined together along their long sides 4a, 4b as shown in FIGS. 1a-1c, the long side 4b of the new board 1′ is pressed against the long side 4a of the previous board 1 as shown in FIG. 1a, so that the locking tongue 20 is introduced into the recess 16. The board 1′ is then angled downwards towards the subfloor 12 according to FIG. 1b. In this connection, the locking tongue 20 enters the recess 16 completely, while the locking element 8 of the strip 6 enters the locking groove 14. During this downward angling the upper part 9 of the locking element 8 can be operative and provide guiding of the new board 1′ towards the previously installed board 1. In the joined position as shown in FIG. 1c, the boards 1, 1′ are locked in both the direction D1 and the direction D2 along their long sides 4a, 4b, but the boards 1, 1′ can be mutually displaced in the longitudinal direction of the joint along the long sides 4a, 4b. 
FIGS. 2a-2c show how the short sides 5a and 5b of the boards 1, 1′ can be mechanically joined in the direction D1 as well as the direction D2 by moving the new board 1′ towards the previously installed board 1 essentially horizontally. Specifically, this can be carried out subsequent to joining the long side of the new board 1′ to a previously installed board 1 in an adjoining row by means of the method according to FIGS. 1a-1c. In the first step in FIG. 2a, bevelled surfaces adjacent to the recess 16 and the locking tongue 20 respectively cooperate such that the strip 6′ is forced to move downwards as a direct result of the bringing together of the short sides 5a, 5b. During the final bringing together of the short sides, the strip 6′ snaps up when the locking element 8′ enters the locking groove 14′, so that the operative locking surfaces 10, 10′ of the locking element 8′ and of the locking groove 14′ will engage each other.
By repeating the steps shown in FIGS. 1a-c and 2a-c, the whole floor can be laid without the use of glue and along all joint edges. Known floorboards of the above-mentioned type are thus mechanically joined usually by first angling them downwards on the long side, and when the long side has been secured, snapping the short sides together by means of horizontal displacement of the new board 1′ along the long side of the previously installed board 1. The boards 1, 1′ can be taken up in the reverse order of laying without causing any damage to the joint, and be laid again. These laying principles are also applicable to the present invention.
For optimal function, subsequent to being joined together, the boards should be capable of assuming a position along their long sides in which a small play can exist between the operative locking surface 10 of the locking element and the operative locking surface 10′ of the locking groove 14. Reference is made to WO 94/26999 for a more detailed description of this play.
In addition to what is known from the above-mentioned patent specifications, a licensee of Välinge Aluminium AB, Norske Skog Flooring AS, Norway (NSF), introduced a laminated floor with mechanical joining according to WO 94/26999 in January 1996 in connection with the Domotex trade fair in Hannover, Germany. This laminated floor, which is marketed under the trademark Alloc®, is 7.2 mm thick and has a 0.6-mm aluminium strip 6 which is mechanically attached on the tongue side. The operative locking surface 10 of the locking element 8 has an inclination (hereinafter termed locking angle) of about 80° to the plane of the board. The vertical connection is designed as a modified tongue-and-groove joint, the term “modified” referring to the possibility of bringing the tongue groove and tongue together by way of angling.
WO 97/47834 (owner Unilin Beeher B. V. , the Netherlands) describes a strip-lock system which has a fibreboard strip and is essentially based on the above known principles. In the corresponding product, “Uniclic®”, which this owner began marketing in the latter part of 1997, one seeks to achieve biasing of the boards. This results in high friction and makes it difficult to angle the boards together and to displace them. The document shows several embodiments of the locking system. The “Uniclic®” product is shown in section in FIG. 4b. 
Other known locking systems for mechanical joining of board materials are described in, for example, GB-A-2,256,023 showing unilateral mechanical joining for providing an expansion joint in a wood panel for outdoor use, and in U.S. Pat. No. 4,426,820 (shown in FIG. 4d) which concerns a mechanical locking system for plastic sports floors, which floor is intentionally designed in such manner that neither displacement of the floorboards along each other nor locking of the short sides of the floorboards by snap action is allowed.
In the autumn of 1998, NSF introduced a 7.2-mm laminated floor with a strip-lock system which comprises a fibreboard strip and is manufactured according to WO 94/26999 and WO 99/66151. This laminated floor is marketed under the trademark “Fiboloc®” and has the cross-section illustrated in FIG. 4a. 
In January 1999, Kronotex GmbH, Germany, introduced a 7.8 mm thick laminated floor with a strip lock under the trademark “Isilock®”. A cross-section of the joint edge portion of this system is shown in FIG. 4c. Also in this floor, the strip is composed of fibreboard and a balancing layer.
During 1999, the mechanical joint system has obtained a strong position on the world market, and some twenty manufacturers have shown, in January 2000, different types of systems which essentially are variants of Fiboloc®, Uniclic® and Isilock®.